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    Subject[PATCH v3 5/7] KVM: X86: MMU: Tune PTE_LIST_EXT to be bigger
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    Currently rmap array element only contains 3 entries.  However for EPT=N there
    could have a lot of guest pages that got tens of even hundreds of rmap entry.

    A normal distribution of a 6G guest (even if idle) shows this with rmap count
    statistics:

    Rmap_Count: 0 1 2-3 4-7 8-15 16-31 32-63 64-127 128-255 256-511 512-1023
    Level=4K: 3089171 49005 14016 1363 235 212 15 7 0 0 0
    Level=2M: 5951 227 0 0 0 0 0 0 0 0 0
    Level=1G: 32 0 0 0 0 0 0 0 0 0 0

    If we do some more fork some pages will grow even larger rmap counts.

    This patch makes PTE_LIST_EXT bigger so it'll be more efficient for the general
    use case of EPT=N as we do list reference less and the loops over PTE_LIST_EXT
    will be slightly more efficient; but still not too large so less waste when
    array not full.

    It should not affecting EPT=Y since EPT normally only has zero or one rmap
    entry for each page, so no array is even allocated.

    With a test case to fork 500 child and recycle them ("./rmap_fork 500" [1]),
    this patch speeds up fork time of about 29%.

    Before: 473.90 (+-5.93%)
    After: 366.10 (+-4.94%)

    [1] https://github.com/xzpeter/clibs/commit/825436f825453de2ea5aaee4bdb1c92281efe5b3

    Signed-off-by: Peter Xu <peterx@redhat.com>
    ---
    arch/x86/kvm/mmu/mmu.c | 4 ++--
    1 file changed, 2 insertions(+), 2 deletions(-)

    diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
    index 16c99f771c9e..c0b452bb5dd9 100644
    --- a/arch/x86/kvm/mmu/mmu.c
    +++ b/arch/x86/kvm/mmu/mmu.c
    @@ -137,8 +137,8 @@ module_param(dbg, bool, 0644);

    #include <trace/events/kvm.h>

    -/* make pte_list_desc fit well in cache line */
    -#define PTE_LIST_EXT 3
    +/* make pte_list_desc fit well in cache lines */
    +#define PTE_LIST_EXT 15

    struct pte_list_desc {
    u64 *sptes[PTE_LIST_EXT];
    --
    2.31.1
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